Method of conditioning anodes

ABSTRACT

Anodes employed in electrolytic cells are conditioned for improved operation by subjecting said anode to an abnormally great current density for a period of time sufficient to deliberately induce anode effect, and then subjecting the anode to an abnormally great voltage for a period of time sufficient to eliminate said anode effect.

United States Patent TO ANODE BUS TO CATHODE BUS 3,461,049 8/1969 Childs 204 /59 3,461,050 8/1969 Childs 204/59 3,471,390 10/1969 Kibby et al 204/245 OTHER REFERENCES H. H. Kellogg, J. of the Electrochem. Soc., Vol. 97, N0. 4, pp. 133-142,4- 1950 Rudge, Influence of Anode Material and Electrolyte Purity on Fluorine Cell Performance," Chemistry and Industry, No. 12,March 19, 1966, pp. 482-488 Primary Examiner.lohn l-I. Mack Assistant Examiner-Neil A. Kaplan Att0rney-Young & Quigg ABSTRACT: Anodes employed in electrolytic cells are conditioned for improved operation by subjecting said anode to an abnormally great current density for a period of time sufficient to deliberately induce anode effect, and then subjecting the anode to an abnormally great voltage for a period of time sufficient to eliminate said anode effect.

I r l j METHOD OF CONDITIONING ANODES This invention relates to a method of conditioning anodes employed in electrolytic cells. In one aspect, this invention relates to an improved method of operating electrolytic cells.

Anode effect is a complex, only partially understood phenomenon which occurs intermittently in many electrochemical processes. Generally speaking, anode effect is a condition which occurs at the anode and which, for all practical purposes, causes the anode to cease operating. The occurrence of anode effect generally requires a corrective action which must be taken to bring the system back to normal operating conditions. Anode effect has been observed in a wide variety of electrolytic cells employing a variety of electrolytes and anode types. For example, it has been observed in cells employing molten salt electrolytes, such as KF-ZHF, and various types of anodes including porous carbon anodes. Anode effect has also been observed in cells having fused salt electrolytes containing halides of lead, silver, cadmium, alkali metals, alkaline earth metals, magnesium, aluminum, and complex electrolytes of aluminum fluorides.

Generally speaking, in the prior art anode effect has been considered highly undesirable, and much effort has been expended to eliminate or at least reduce the frequency of its occurrence. However, we have now discovered a method of operating electrolytic cells wherein it is desirable to deliberately induce anode effect. We have discovered that by deliberately inducing the phenomenon of anode effect we can realize or obtain marked benefits in cell operation. These benefits include a marked reduction in the amount of cell byproducts and also usually includes a marked reduction in the operating voltage required after the deliberately induced anode effect has been eliminated.

Thus, broadly speaking, the present invention provides a method for conditioning anodes for improved operation in electrolytic cells. In said method the anode is conditioned by deliberately inducing anode effect and then eliminating said anode effect. Preferably, said anode effect is induced prior to or during'the initial period of operation of the cell so as to realize the advantages of said benefits for the maximum period of time.

An object of this invention is to provide a method for conditioning anodes for improved operation in electrolytic cells. Another object of this invention is to provide an improved method of operating electrolytic cells. Another object of this invention is to provide an improved method of operating electrolytic cells wherein less undesirable byproducts are produced. Another object of this invention is to provide an improved method of operating electrolytic cells wherein less voltage is required. Another object of this invention is to provide an improved method for initiating the operation of electrolytic cells. Another object of the invention is to provide an improved method for the electrochemical fluorination of organic compounds. Another object of this invention is to provide an improved method for electrochemically fluorinating inorganic compounds. Other aspects, objects, and advantages of the invention will be apparent to those skilled in the art in view of this disclosure.

Thus, according to the invention, there is provided a method of conditioning an anode for improved operation in an electrolysis process normally carried out by passing an electric current, at current density and voltage values normal for said process, through a molten electrolyte contained in an electrolytic cell having an anode and a cathode disposed in said electrolyte, which method comprises: subjecting said anode to an abnormally high current density for a period of time sufficient to deliberately induce anode effect at said anode; and then subjecting said anode to an abnormally high voltage for a period of time sufficient to eliminate said anode effect.

It will be noted that in the practice of the invention anode effect is deliberately induced. This is accomplished by increasing the current density to an abnormally high value. Generally speaking, in inducing said anode effect the current density will be increased an amount within the range of from about 25 to about 200 percent of the normal current density. Frequently,

an increase within the range of about 25 to about I00 percent will be sufficient. However, there is no real numerical limit on the amount of current density increase. As a general rule, the smaller amounts of current density increase are preferred. Amounts of current density increase outside the above specified ranges can be employed. The minimum amount is the smallest amount which is necessary to deliberately induce the anode effect. The maximum amount will be limited by the limit of the particular power supply available. The amount of normal current density and the amount of current density increase employed will depend upon the particular system being employed, Some anode materials are more susceptible to anode effect than other materials. For example, a dense porous carbon is more susceptible than a loose or more porous carbon. The period of time to which the anode is subjected to the increased current density will be a period which is just sufficient to induce the anode effect. This period of time will preferably be short, generally from a few seconds to a few minutes, e.g. 0.1 to 10 minutes, once the required level of increased current density has been reached. Thus, numerically speaking, the period of time to which the anode is subjected to the increased current density can vary over a wide range, including values outside said range.

The amount of voltage increase which is employed to correct the deliberately induced anode effect will usually be within the range of from about 3 to about l0 times the voltage employed in the normal operation of the cell. The amount of normal voltage and the amount of voltage increase employed will depend upon the particular system being employed. Frequently, said voltage increase will be within the range of about 3 to about 7 times said normal voltage. However, voltages outside said range can also be employed in the practice of the invention. These high voltages are preferably employed in the minimum amount necessary and for the minimum period of time sufficient to eliminate the deliberately induced anode effect. Minimum voltage and minimum time are employed so as to avoid, or reduce to a minimum, increased heating of the cell elements. High voltages will frequently be destructive to the material from which the anode is fabricated, particularly when the anode is fabricated from a porous material such as porous carbon. Said high voltages will usually be applied for a period of time within the range of from about 0.5 to 5, preferably 0.5 to 3, minutes. However, in the practice of the invention said high voltages can be applied for periods of time outside said ranges, if necessary.

The step of deliberately inducing anode effect can be carried out at any time during the operation of the electrochemical process in which the invention is being applied. In one method of operation in accordance with the invention, a combination of four steps is carried out. In general, these four steps are: (l) a period of cell operation under normal conditions; (2) the deliberate inducing of anode effect by increasing current density; (3) the elimination of said deliberately induced anode effect by increasing the voltage; and (4) decreasing the cell voltage to a value sufficient to maintain normal current density. Step (l) is preferably carried out during the first portion of the cell operating time, e.g., within a period of time ranging from 1 or 2 minutes up to about 48 hours. Preferably, in most instances, said deliberately induced anode effect will be carried out during the first 24 hours of cell operation. However, in some instances, it will be desirable to deliberately induce the anode effect at the start of cell operation, i.e., at zero period of time of normal operation. Thus, in some instances, said step (1) can be eliminated. Step (4) can be carried out in more than one manner. One simple method comprises merely opening the electrical circuit which, of course, will cause the voltage to decrease to zero. After a few seconds the cell, if the anode effect has been corrected, will operate at normal current density and usually at a lower voltage level than prior to said anode effect. The other method comprises a reduction of voltage to the desired value without an interruption in current flow.

It is not necessary that the anode be given the anode effect inducing step of the invention while it is located in the production cell where it is normally or ultimately employed. If desired, this treatment of the anode can be carried out in a separate cell which is operated under conditions approximating the operating conditions of the production cell, e.g., the same general conditions with respect to electrolyte, current density, voltage, etc. The thus treated anode can then be stored for future use or transferred to the regular production cell.

lf desired, the anode effect inducing step of the invention can be carried out incrementally, e.g., stepwise. For example, in one method of practicing the invention, an anode to be conditioned can be lowered into the current-conducting electrolyte in stages or steps. in this manner only a relatively small portion, e.g., from about 0.l to about 0.5, preferably about 0.1 to about 0.3, of the anode is treated at one time. Thus, the conditioning treatment of the invention can be started in a cell with only a small portion of the anode extending into an electrolyte being electrolyzed under normal current and voltage conditions. This subjects said small portion to an abnormally high current density and anode effect is quickly induced on the immersed portion of the anode. A second increment of the anode is then immersed and anode effect is quickly induced thereon. This incremental or stepwise treatment is. continued until anode effect has been induced on the entire anode. Preferably, the voltage is then increased to an abnormally high value sufficient to eliminate said anode effect from the entire anode, as described elsewhere herein. The above-described incremental or stepwise inducing of anode effect can be carried out in the regular production cell where the anode is, or is to be, employed, or it can be carried out in a separate cell. if a separate cell is employed, the step of correcting or eliminating the deliberately induced anode effect can be carried out therein, or can be delayed until the anode is transferred to the production cell. The above-described incremental or stepwise inducing of the anode effect has great advantages in power requirements, particularly in systems when relatively large anodes are being employed. Less power is required for each of the incremental steps than for an entire anode. Thus, a given system can be operated with much smaller potential power supply.

The invention is applicable to electrochemical processes wherein anode effect occurs. The invention is particularly valuable in the electrochemical fluorination of organic materials using a current-conducting essentially anhydrous hydrogen fluoride electrolyte. For purposes of convenience, and not by way of limitation, the invention will be further described as applied to electrochemical fluorination. Various processes for carrying out electrochemical fluorination reactions are known. in one presently preferred process a current-conducting essentially anhydrous liquid hydrogen fluoride electrolyte is electrolyzed in an electrolysis cell provided with a cathode and a porous anode (preferably porous carbon), a fluorinatable organic compound is introduced into the pores of said anode and at least a portion of said organic compound is at least partially fluorinated within the pores of said anode, and fluorinated compound products are recovered from the cell.

Very few organic compounds are resistant to fluorination. Consequently, a wide variety of feed materials, both normally liquid and normally gaseous compounds, can be used as feedstocks in said process. Organic compounds which are normally gaseous or which can be introduced in gaseous state into the pores of a porous anode under the conditions employed in the electrolysis cell, and which are capable of reacting with fluorine, are presently preferred as starting materials. However, starting materials which are introduced into the pores of the anode in liquid state can also be used. Generally speaking, desirable organic starting materials which can be used are those containing from one to eight, preferably one to six, carbon atoms per molecule. However, reactants which contain more than six or eight carbon atoms can also be used. lf desired, suitable feed materials having boiling points above cell operating temperatures can be passed into the pores of the porous anode in gaseous state by utilizing a suitable carrier gas. Thus, a suitable carrier gas can be saturated with the feed reactant (as by bubbling said carrier gas through the liquid reactant), and then passing the saturated carrier gas into the pores of the porous anode. Suitable carrier gases include the inert gases such as helium, argon, krypton, neon, xenon, nitrogen, etc. Perfluorocarbons containing from one to eight, preferably one to four, carbon atoms per molecule such as tetrafluoromethane, hexafluoroethane, perfluoropropane, the perfluorobutanes, the perfluorohexanes, and the perfluorooctanes can also be used as carrier gases. said carrier gases can also be used as diluents for the feedstocks which are normally gaseous at cell operating conditions. Some general types of starting materials which can be used include, among others, the following: alkanes, alkenes, alkynes, amines, ethers, esters, mercaptans, nitriles, alcohols, aromatic compounds, and partially halogenated compounds of both the aliphatic and aromatic series. It will be understood that the above-named types of compounds can be either straight chain, branched chain, or cyclic compounds. The presently more preferred starting materials are the normally gaseous organic compounds, and particularly the saturated and unsaturated hydrocarbons, containing from one to four carbon atoms per molecule.

Since fluorine is so reactive, no list of practical length could include all starting materials which can be used. However, representative examples of the above-described starting materials include, among other, the following: methane; ethane; propane; butane; isobutane; pentane; n-hexane; n-octane; cyclopropane; cyclopentane; cyclohexane; cyclooctane; 1,2-dichloroethane; l-fluoro-2-chloro-3 -methylheptane; ethylene; propylene; cyclobutene; cyclohexene; Z-methylpentenel; 2,3-dimethylehexene-2; butadiene; vinyl chloride; 3- fluoropropylene; acetylene; methylacetylene; vinylacetylene; 3,3-dimethylpentyne-2; allyl chloride; methylamine; ethylamine; diethylamine; 2-amino-3-ethylpentane; 3- bromopropylamine; triethylamine; dimethyl ether; diethyl ether; methyl ethyl ether; methyl vinyl ether; 2-iodoethyl ether; din-propyl ether; methyl formate; methyl acetate; ethyl butyrate; ethyl formate; n-amyl acetate; methyl 2- chloroacetate; methyl mercaptan; ethyl mercaptan; n-propyl mercaptan; 2-mercaptohexane; 2-methyl-3-mercaptoheptane; acetonitrile; propionitrile; n-butyronitrile; acrylonitrile; n-hexanonitrile; methanol; ethanol; isopropanol; n-hexanol; 2,2- dimethlhexanol-Ii; n-butanol; ethylenebromohydrin; benzene; toluene; cumene; o-xylene; p-xylene; and monochlorobenzene.

Inorganic materials such as carbon monoxide and oxygen can also be fluorinated as described herein to yield carbonyl fluoride and oxygen difluoride, respectively. Although the hydrogen fluoride electrolyte can contain small amounts of water, such as up to about 5 weight percent, it is preferred that said electrolyte be essentially anhydrous. Generally speaking, it is preferred that said electrolyte contain not more than about 0.1 weight percent water. However, commercial anhydrous liquid hydrogen fluoride which normally contains dissolved water in amounts ranging from a trace (less than 0.1 weight percent) up to about 1 percent by weight can be used. Thus, as used herein and in the claims, the term essentially anhydrous liquid hydrogen fluoride, unless otherwise specified, includes liquid hydrogen fluoride which can contain water not exceeding up to about 1 weight percent. As the electrolysis reaction proceeds, any water contained in the hydrogen fluoride electrolyte is slowly decomposed and said electrolyte concomitantly approaches the anhydrous state. The hydrogen fluoride electrolyte is consumed in the reaction and must be either continuously or intennittentiy placed in the cell.

Pure anhydrous liquid hydrogen fluoride is nonconductive. The essentially anhydrous liquid hydrogen fluorides described above have a low conductivity which, generally speaking, is lower than desired for practical operation. To provide adequate conductivity in the electrolyte, and to reduce the hydrogen fluoride vapor pressure at cell operating conditions, an inorganic additive can be incorporated in the electrolyte. Examples of suitable additives are inorganic compounds which are soluble in liquid hydrogen fluoride and provide effective electrolytic conductivity. The presently preferred additives are the alkali metal (sodium, potassium, lithium, rubidium, and cesium) fluorides and ammonium fluoride. Other additives which can be employed are sulfuric acid and phosphoric acid. Potassium fluoride is the presently most preferred additive. Said additives can be utilized in any suitable molar ratio of additive to hydrogen fluoride within the range of from 1:45 to lzl, preferably 1:4 to 1:2. The presently most preferred electrolytes are those which correspond approximately to the formulas KF-ZHF, KF3HF, or KF-4HF. In general, said additives are not consumed in the process and can be used indefinitely. Said additives are frequently referred to as conductivity additives for convenience.

The cell body and the electrodes in the cell must be fabricated of materials which are resistant to the action of the contents of the cell under the reaction conditions. Materials such as steel, iron, nickel, polytetrafluoroethylene (Teflon), carbon, and the like, can be employed for the cell body. The cathode can be fabricated in any suitable shape or design and can be made of any suitable conducting material such as iron, steel, nickel, alloys of said metals, and carbon. The anode preferably will comprise a porous element. Said anode can be fabricated from any suitable conducting material which is compatible with the system, e.g. nickel, iron, various metal alloys, and carbon, which is not wetted by the electrolyte. By not wetted is meant that the contact angle between the electrolyte and the anode must exceed 90 in order that anticapillary forces will prevent substantial invasion of the small pores of the anode by the electrolyte. Porous carbon, which is economical and readily available in ordinary channels of commerce, is presently preferred for the porous element of said anode. Porous carbon impregnated with a suitable metal such as nickel can also be used as the anode. Various grades of porous carbon can be used in the practice of the invention. lt is preferred to employ porous carbon which has been made from carbon produced by pyrolysis, and not graphitic carbon. Two types of commercially available porous carbon are those known commercially as Stackpole 139 and National Carbon Grade 60. Said Stackpole 139 carbon has a pore volume of about 0.2 to about 0.3 cc. per gram with the pore diameters ranging from 0.1 to microns in diameter. Said National Carbon Grade 60 has a pore volume of about 0.3 to about 0.5 cc. per gram with the pore diameters ranging form 10 to 60 microns in diameter. The actual values of said pore volumes will depend upon the specific method employed for determining same. Thus, preferred porous carbons for fabricating the preferred anodes include those having a pore volume within the range of about 0.2 to about 0.5 cc. per gram with the pores ranging from 0.1 to 60 microns in diameter.

When the porous element of said anode is porous carbon, said anode can, if desired, comprise another conducting element which is in contact with said porous carbon element. Said other conducting element can be fabricated from any suitable conducting material which is compatible with the system, e.g., nickel, iron, cobalt, steel (including the various carbon steels and the various stainless steels), and alloys of nickel with other metals which contain at least 5 weight percent nickel. lncluded among said alloys of nickel are: alloys of nickel with titanium; alloys of nickel with copper, such as Monel; the various Hastelloys; the various lnconel alloys; and the various Chlorimet alloys. Some of said metals and alloys are more compatible with the system than others are, but all are operable within the scope of the invention. The presently most preferred metals for utilization as said other conducting element are essentially pure nickel, e.g., the various commercially available grades of nickel metal, and the high nickel alloys, e.g., those alloys of nickel containing at least 50 weight percent nickel.

Said anode or said combination anode of porous carbon and another conducting element can be fabricated in any suitable shape or design, but must be arranged or provided with a suitable means for introducing the feed reactant material into the pores of the porous element thereof.

Except as described above, any convenient cell configuration or electrode arrangement can be employed. The cell must be provided with a vent or vents through which byproduct hydrogen can escape and through which volatile cell products can be removed and recovered. If desired or necessary, a drain can be provided on the bottom of the cell. The cell may contain an ion permeable membrane or divider for dividing the cell into an anode compartment and a cathode compartment. It is sometimes desirable to employ such a divider to prevent hydrogen generated at the cathode from mixing and reacting with the fluorinated product produced at the anode. Any conventionally known resistant divider material can be employed for this purpose. When the anode products are withdrawn from the cell through a conduit means directly connected to the anode, as described further hereinafter, said divider can be omitted.

The electrochemical fluorination can be effectively and conveniently carried out over a broad range of temperatures and pressures limited only by the freezing point and the vapor pressure of the electrolyte. Generally speaking, the fluorination process can be carried out at temperatures within the range of from to 500 C. at which the vapor pressure of the electrolyte is not excessive, e.g., less than 250 mm. Hg. It is preferred to operate at temperatures such that the vapor pressure of the electrolyte is less than about 50 mm. Hg. A presently preferred range of temperature is from about 60 to about C.

Pressures substantially above or below atmospheric can be employed if desired, depending upon the vapor pressure of the electrolyte as discussed above. In all instances, the cell pressure will be sufficient to maintain the electrolyte in liquid phase. Generally speaking, the process is conveniently carried out at substantially atmospheric pressure. The operating conditions of temperature and pressure within the limitations discussed above are not critical and are essentially independent of the type of feed employed in the process.

The rate of direct current flow through the cell is maintained at a rate which will give the highest practical current densities for the electrodes employed. Current densities within the range of from 30 to 1,000, or more, preferably 50 to 500 milliamps per square centimeter of anode geometric surface area can be used. Current densities less than 30 milliamps per square centimeter of anode geometric surface area are not practical because the rate of fluorination is too slow. The voltage which is normally employed will vary depending upon the particular cell configuration employed and the current density employed. In all cases, under normal operating conditions, however, the cell voltage or potential will be less than that required to evolve or generate free or elemental fluorine. Voltages in the range of from 4 to 12 volts are typical. The maximum normal voltage will not exceed 20 volts per unit cell. Thus, as a guide, voltages in the range OH to 20 volts per unit cell are normally used. The term anode geometric surface" refers to the outer geometric surface area of the porous carbon element of the anode which is exposed to electrolyte and does not include the pore surfaces of said porous element. For example, in FIG. 1 the anode geometric surface is the ver tical cylindrical sidewall of the porous carbon element 12.

The feed rate of the fluorinatable material (including any carrier or diluent gas) being introduced into the pores of the porous carbon element of the anode is an important process variable in that, for a given current flow or current density, the feed rate controls the degree of conversion. Similarly, for a given feed rate, the amount of current flow or current density can be employed to control the degree of conversion. Feed rates which can be employed will preferably be in the range of from 0.5 to 10 milliliters per minute per square centimeter of anode geometric surface area. With the higher feed rates,

higher current density and current rates are employed. Since the anode can have a wide variety of geometrical shapes, which will affect the geometrical surface area, a sometimes more useful way of expressing the feed rate is in terms of anode cross-sectional area (taken perpendicular to the direction of flow).

The actual feed rate employed will depend upon the type of porous material, e.g., porous carbon, used in fabricating the porous element of the anode as well as several other factors including the nature of the feedstock, the conversion desired, current density, etc., because all these factors are interrelated and a change in one will afiect the others. The feed race will be such that the feedstock is passed into the pores of the anode, and into contact with the fluorinating species therein, at a flow rate such that the inlet pressure of said feedstock into said pores is essentially less than the sum of (a) the hydrostatic pressure of the electrolyte at the level of entry of feedstock into said pores and (b) the exit pressure of any unreacted feedstock and fluorinated products from said pores into the electrolyte. Said exit pressure is defined as the pressure required to form a bubble on the outer surface of the anode and break said bubble away from said surface. Said exit pressure is independent of hydrostatic pressure. Under these preferred flow rate conditions, there is established a pressure balance between the feedstock entering the pores of the anode from one direction and electrolyte attempting to enter the pores from another and opposing direction. This pressure balance provides an important and distinguishing feature in that essentially none of the feed leaves the anode to form bubbles which escape into the main body of the electrolyte. Essentially all of the feedstock travels within the carbon anode via the pores therein until it reaches a collection zone within the anode from which it is removed via a conduit, or until it exits from the anode at a point above the surface of the electrolyte.

The more permeable carbons will permit higher flow rates than the less permeable carbons. Any suitable porous carbon which will permit operation within the limits of the abovedescribed pressure balance can be employed in fabricating the porous element of the anode. Thus, broadly speaking, porous carbons having a permeability within the range of from 0.5 to 75 darcys and average pore diameters within the range of from l to 150 microns can be employed. Generally speaking, carbons having a permeability within the range of from about 2 to about 30 darcys and an average pore diameter within the range of from about to about 75 microns are more preferred.

Similarly, anode shapes, anode dimensions, and manner of disposal of the anode in the electrolyte will also have a bearing on the flow rate. Thus, owing to the many different types of carbon which can be employed and the almost infinite number of combinations of anode shapes, dimensions, and methods of disposal of the anode in the electrolyte, there are no really fixed numerical limits on the flow rates which can be used. Broadly speaking, the upper limit on the flow rate will be that at which "breakout of feedstock and/or fluorinated product begins along the immersed portion of the anode when the anode is provided with an internal collection zone as in FIG. 1, or the top of the anode is above the surface of the electrolyte as in FIG. 2 when cap 38 is omitted. Herein and in the claims, unless otherwise specified, breakout" is defined as the for mation of bubbles of feedstock and/or fluorinated product on the outer immersed surface of the anode with subsequent detachment of said bubbles wherein they pass into the main body of the electrolyte. Broadly speaking, the lower limit of the feed rate will be determined by the requirement to supply the minimum amount of feedstock sufficient to prevent evolution of free fluorine. As a practical guide to those skilled in the art, the flow rates can be within the range of from 3 to 600, preferably 12 to 240, cc. per minute per square centimeter of cross-sectional area (taken perpendicular to the direction of flow).

The above-described pressure balance will permit some invasion of the pores of the anode by the hydrogen fluoride electrolyte. The amount of said invasion will depend upon the inlet pressure of the feedstock and the pore size. The larger size pores are more readily invaded. It has been found that porous carbon anodes as described herein can be successfully operated when up to 40 to 50 percent of the pores have been invaded by liquid I-IF electrolyte.

The feed material and the products obtained therefrom are retained in the cell for a period of time which is generally less than 1 minute. The fluorinated products and the unconverted feed are passed from the cell and then are subjected to conventional separation techniques such as fractionation, solvent extraction, adsorption, and the like, for separation of unconverted feed and reaction products. Unconverted or insufficiently converted feed materials can be recycled to the cell for the production of more highly fluorinated products, if desired. Perfluorinated products, or other products which have been too highly fluorinated, can be burned to recover hydrogen fluoride which can be returned to the cell, if desired. Byproduct hydrogen can be burned to provide heat energy or can be utilized in hydrogen-consuming processes such as hydrogenation, etc.

FIG. 1 is a view, partly in cross section, diagrammatically illustrating one type of anode and cell arrangement to which the invention is applicable.

FIG. 2 is a view, partly in cross section, diagrammatically illustrating another anode and cell arrangement to which the invention is applicable.

In FIG. 1 a generally tubular conductor and current collector 10 provides support for porous element or member 12. Said porous element or member 12 has the general shape of a hollow tube, closed at one end thereof, and open at the other end. Preferably, the bottom of said porous element is sealed with a resistant cement material 14 such as Fluoroseal. One end portion 16 of said tubular conductor 10 is formed with a reduced cross section. A Teflon tape seal material 11 coats the lower portion of conductor 10. A shoulder surrounds the upper end of said reduced cross section portion 16. If desired, a suitable gasket material 18 can be inserted between said shoulder and the upper end of porous element 12. A vent 20 extends through the wall of said portion of reduced cross section 16. Said vent 20 can comprise a plurality of holes drilled through portion 16 into communication with annular space 22. A first conduit 24 extends through said tubular conductor 10, out the lower portion 16 thereof, and forms said annular space 22 within said tubular conductor. A metal plug 26 is mounted on and surrounds the lower end portion of said conduit 24. Said porous element 12 is mounted on said reduced cross section portion 16 against said shoulder and gasket material 18, covers said vent 20, and surrounds said plug 26. An exit vent conduit 28, in communication with said annular space 22, extends outwardly from the upper portion of said conductor and current collector 10. Thus, there is provided a first surface of porous element 12 which comprises the wall surface of chamber 30, which is formed below plug 26, for the introduction of feed reactant into the pores of porous element 12. The upper portion of the inner wall of porous element l2 which is exposed by vent 20 and thus in communication with annular space 22 comprises a second surface for withdrawal of products and unreacted feedstock from within the pores of porous element 12. A circular cathode 32 which can be formed of 20X20 mesh mild steel screen surrounds said anode. Said anode and cathode are suitably disposed in a suitable cell container 34 containing a suitable electrolyte, the level of which is indicated at 36. Conduit 35 provides means for withdrawal of cell products such as cathode products. It will be understood that current collector l0 and the lead 37 to the cathode bus are suitably insulated from the top wall of container 34.

In FIG. 2, the porous anode element 10 also has the general shape of a hollow tube, closed at one end thereof, and open at the other end. Preferably, the outside bottom surface of said porous element 10 is sealed as in FIG. l. A generally tubular cap 33 formed of a suitable electrolyte resistant material such as Teflon is positioned above the upper end of said porous element 10. A generally tubular flange 40 depends from the lower side of said cap and surrounds the upper end portion of said porous element 10. A shoulder 42 formed on the inner wall of said flange 40 is in contact with the upper end of said porous element. Said cap 38 with its depending flange 40 and shoulder 42 thus forms a gas dome which covers and surrounds the top of said porous element 10 and provides means for supporting same. A first conduit 44 extends through said cap 38 and into the interior of hollow porous element 10. Preferably, said contact between shoulder 42 and the top of porous element 10 is only a point" contact. However, shoulder 42 can overlap a portion of the top of porous element 10. Preferably, the contact between said shoulder 42 and porous element 10 does not fonn a seal. With the structure shown, the electrolyte itself serves as a seal.

The lower portion 46 of said first conduit 44 is in close fitting relationship with the inner wall of porous element 10. If desired, said lower end portion 46 can be threaded into said inner wall. Said lower end portion 46 thus forms a chamber 30 at the lower end thereof which is defined by the lower inner wall of porous element 10 and comprises a first surface for introducing feedstock into the pores of said porous element 10. Said first conduit 44 and its lower end portion 46 also comprise a conductor and current collector, similarly as in FIG. 1. ln passing through cap 38 said first conduit 44 forms an annular space 48 within said cap and above the upper end of porous element 10. An exit vent conduit 50 is in communication with said annular space 39. The upper end surface of porous element 10 comprises a second surface for withdrawal of product and unreacted feedstock from within the pores of said porous element and passing said withdrawn materials through a second conduit means comprising said annular space 48 and exit vent conduit 50. The anode illustrated in FIG. 2 can be surrounded by a suitable cathode and disposed in a suitable container, similarly as illustrated in FIG. 1. If desired, and particularly where it is not desired to collect the anode products separately, the cap 38 can be omitted in which case the anode products can exit from the upper surface of element 10, and are collected in and withdrawn from the space in the cell above the surface of the electrolyte.

In the above-described presently preferred electrochemical fluorination process the fluorination is carried out within the pores of the porous carbon anode. However, the present invention is not limited to being employed in processes of this type. The invention is also applicable to those processes wherein the feedstock is passed into the anode at such a rate and under pressure conditions such that it passes through the anode and bubbles out into direct contact with and through the main body of electrolyte, and converted anode products and unconverted feedstock are withdrawn from the space above the electrolyte along with cathode products for subsequent separation. An anode suitable for use in such a process can be provided by omitting elements 16 and 26 in the anode of FIG. I or omitting the lower as or 34 of element 46 in the anode of FIG. 2. The invention is also applicable to those processes wherein solid anodes, either porous or nonporous and formed of either cylindrical, rectangular, or other convenient shape, are employed.

The invention is also applicable to electrochemical fluorination processes employing electrolytes other than essentially anhydrous hydrogen fluoride, and/or other types of anodes. For example, Radimer in US. Pat. No. 2,841,544 describes an electrochemical fluorination process using molten salts such as a eutectic mixture of any two of sodium fluoride, potassium fluoride, calcium fluoride, magnesium fluoride, or aluminum fluoride, etc., as electrolyte. Simons in US. Pat. No. 2,519,983 and Ashley et al. in U.S. Pat. No. 3,298,940 describe other electrochemical fluorination processes to which the present invention is applicable.

The following examples will serve to further illustrate the invention.

it EXAMPIEI Ethylene dichloride was electrochemically fluorinated in a cell using molten KF'ZHF electrolyte maintained at 89-90 C. as the electrolyte. Hydrogen fluoride was replenished as it was consumed in the reaction. The anode comprised a porous carbon cylinder having a length of 6 inches, an outside diameter of 1% inches, and an inside diameter of about a inch. Said anode was essentially like the anode illustrated in FIG. 2 except that the cap 38 was omitted. A hollow copper rod was threaded into the anode to within about 15 inch of the bottom of serve as the current collector and as the ethylene dichloride feed inlet. The anode was fashioned from a commercial grade porous carbon having an efiective porosity of about 48 percent with an average pore diameter of about 58 microns. In its installed position the anode was immersed in the electrolyte to a depth of about 3% Inches. The cathode was a mild steel wire gauze, circular In shape, and situated around the anode. No separator was employed to separate the anode products from the cathode products. FIG. I is a diagrammatic illustration of the cell arrangement employed.

The electrochemical conversion was started at a current density of 200 ma/cmF, a total current of 20 amps, a voltage of about 7.8 volts, and an ethylene dichloride feed rate of 50 volume cc./hr. After about 1 hour, the effluent from the electrolytic cell was sampled and analyzed. The analysis showed that 27.2 mol percent of the products were undesirable byproduct monochloro and trichloro derivatives.

After being onstream for about 2 hours, the anode underwent anode effect and electrolysis essentially ceased. The voltage was increased to about 35 volts. After 1% minutes at the elevated voltage the anode effect was eliminated and the voltage was dropped back to about 8 volts as the cell came back into normal operation. About 48 hours later, the effluent from the cell was sampled and analyzed again. This analysis showed only 14.5 mol percent of the products were the undesirable monochloro and trichloro byproducts. Thus, these data show the beneficial effect, in regard to byproduct formation, resulting from an anode efiect-anode effect elimination sequence in the electrochemical fluorination of ethylene dichloride.

EXAMPLE Il Another run, similar to that of example I, was carried out employing the same cell and apparatus, but with a new but otherwise identical porous carbon anode. Ethylene dichloride was again the feed material, being passed into the anode at a feed rate of 47 volume cc./hr. The temperature of the electrolyte was 96 C. The electrolytic cell was started up and when brought to operating conditions was operating at a current density of 200 ma/cm. a total current of 20 amps, and a voltage of about 9.0 volts.

After being onstream about 20 hours, anode effect was deliberately induced by increasing the current flow to 25 amps for a period of about 3 minutes. The anode effect was then eliminated by the application of high voltage for a short period of time and the operation of the cell was resumed at 20 amps. It was found that this current level could now be maintained with a voltage of only 7.6 volts. About 4 hours later the effluent from the cell was sampled and analyzed. The analysis showed the presence of only l 1.1 mol percent of the undesirable monochloro and trichloro byproducts. Thus, these data show that the deliberate inducing of an anode effect-anode effect elimination sequence can be employed to reduce undesirable byproduct formation in the electrochemical fluorination of ethylene dichloride.

EXAMPLE III Another run for the electrochemical fluorination of ethylene dichloride was carried out. In this run the cell as sembly and electrolyte employed were essentially the same as that described above in example I except that the anode comprised a solid bar of porous carbon having a length of 14 inches and an outside diameter of 1% inches. Said anode was fitted for the introduction of the ethylene dichloride feedstock into a recess on the bottom of the anode. In operation the anode was immersed in the electrolyte to a depth of about 12 inches.

The electrochemical conversion was started at a current density of 200 ma/cmF, a total current of 67 amperes, a voltage of about 9.4 volts, and an ethylene dichloride feed rate of about 98 volume milliliters per hour. The cell temperature was about 100 C. After a few hours operation the effluent from the cell was sampled and analyzed. The analysis showed that 32.6 mol percent of the products were undesirable byproduct monochloro and trichloro derivatives. After being onstream for about 3 days, anode effect was deliberately induced at a current density of 330 ma/cmF. The anode effect was eliminated by the application of an abnormally high voltage of about 60 volts for a short period of time, and operation of the cell was resumed at an anode current density of 200 ma/cm). After about l'z hours operation at the resumed normal operating conditions, the cell effluent was again sampled and analyzed. This analysis showed the presence of only 19.6 mol percent of the undesirable monochloro and trichloro byproducts.

EXAMPLE IV A rectangular porous carbon anode (National Carbon PC 45) measuring 14 inches long, 3 inches wide, and 1% inches thick was fitted in a conventional manner with a 56-inch diameter copper pin as a current collector. Said anode was then subjected to a deliberately induced anode effect in a conventional cell employing a molten KF'ZHF electrolyte and an iron cathode.

Said anode effect was induced incrementally in the following manner. First, a potential of 28-30 volts was applied between the anode and cathode while the anode was still out of the electrolyte. The anode was then incrementally immersed, 1 inch at a time, into the electorlyte-being held at each level long enough, generally less than a minute, to induce anode effect for that increment of the anode. As each new section was immersed, the current was initially about 50 to 70 amps. As the section became subject to anode effect, the current decreased to about amps. The anode was treated in this manner up to a total immersion of 12 inches. It was removed from the electrolyte while remaining in the polarized or induced anode effect state;

When it is desired to depolarize the anode or eliminate the anode effect, the anode is again immersed in a similar electrolyte and connected to a suitable power supply. The power is turned on, and the voltage is increased to an abnormally high value of about 45 to 60 volts. After about 3 minutes at this voltage level, the power supply is turned off momentarily. it is then restarted after a few seconds and the cell operates normally at about 200 ma/cm. current density at a normal voltage, e.g., about 8 to 10 volts.

While the invention has been described above with particular reference to processes comprising the electrochemical fluorination of organic compounds, the invention is not so limited. The invention has application in other electrolysis processes wherein the phenomenon of anode effect is encountered. For example, the invention is applicable to electrolytic processes for the production of fluorine comprising electrolysis of fused salt baths such as KF'ZHF employing carbon anodes. Such processes are well known in the art.

While certain embodiments of the invention have been described for illustrative purposes, the invention obviousiy is not limited thereto. Various other modifications will be apparent to those skilled in the art in view of this disclosure. Such modifications are within the spirit and scope of the invention.

We claim:

1. A method of conditioning an anode for improved operation in an electrochemical process for fluorinating a fluorinatable material which is normally carried out by passing an electric current, at current density and voltage values normal for said process, through an essentially anhydrous current-conducting hydrogen fluoride electrolyte contained in an electrolytic cell having an anode and a cathode disposed in said eiectrolyte, and wherein said anode when disposed in said electrolyte is subject to anode efiect, which method comprises: prior to or during substantially normal operation of said cell, deliberately inducing anode effect at said anode by subjecting said anode to an abnormal current density sufficiently high to produce anode efiect, for a period of time sufficient to induce said anode effect; and then eliminating said deliberately induced anode effect by subjecting said anode to an abnormal voltage sufficiently high to eliminate said anode effect, for a period of time sufficient to eliminate said anode effect.

2. A method in accordance with claim 1 wherein said anode effect is deliberately induced and then eliminated prior to initiating nonnal operation of said cell.

3. A method in accordance with claim 1, comprising, in combination, the steps of: initially passing said current through said electrolyte at a normal current density and a normal voltage sufficient for carrying out said process; then increasing said current density to an abnormally high value which is greater than said normal current density for a period of time sufficient to deliberately induce said anode eflect at said anode; then increasing said cell voltage to an abnormally high value which is greater than said nonnal voltage for a period of time sufficient to eliminate said anode effect; and returning said cell voltage to a normal value sufficient to maintain said normal current density.

4. A method in accordance with claim 3 wherein: said electric current is initially passed through said electrolyte at said normal current density and voltage values for a period of time sufficient to establish normal operation of said process; said current density is then increased to an abnormal value within the range of from about 25 to about 200 percent greater than said normal value; and said voltage is then increased to an abnormal value within the range of from about three to about 10 times said normal value.

5. A method in accordance with claim 4 wherein: said initial period of time is within the range of from a few minutes to about 1 hour; said current density is then increased for a period of time within the range of from 0.1 to l0 minutes; and said voltage is then increased for a period of time within the range of from 0.5 to 5 minutes.

6. A method according to claim 1 wherein said anode is a porous anode, said fluorinatable material is an organic compound, and said electrolysis process comprises passing said organic compound into the pores of said anode, and recovering fluorinated product from an effluent stream from said cell.

7. A method according to claim 6 wherein said anode is a porous carbon anode and said organic compound is fluorinated within the pores of said anode.

8. A method according to claim 7 wherein said organic compound is ethylene dichloride.

9. A method according to claim 1 wherein said anode is incrementally subjected to said abnormally high current density.

10. A method according to claim 1 comprising, in combination, the steps of:

applying an electrical potential between said anode and said cathode while said anode is not immersed in said electrolyte;

immersing a first portion of said anode in said electrolyte so as to subject said portion to said abnormally high current density and induce said anode effect thereon;

then immersing a succeeding portion or portions of said anode in said electrolyte so as to subject each said succeeding portion to said abnormally high current density and induce said anode effect thereon; and

then subjecting said anode to said abnormally high voltage so as to eliminate said anode effect.

11. A method according to claim wherein each said portion of said anode is a small portion comprising from about 0.1 to about 0.3 ofthe anode.

12. A method according to claim 1 comprising, in combination, the steps of:

initially immersing only a first portion of said anode in said electrolyte so as to subject said portion to said abnormally high current density and induce said anode effect thereon;

then immersing a succeeding portion or portions of said anode in said electrolyte so as to subject each said succeeding portion to said abnormally high current density and induce said anode effect thereon; and

then subjecting said anode to said abnonnally high voltage so as to eliminate said anode effect.

13. A method according to claim 12 wherein each said portion of said anode is a small portion comprising from about 0.1 to about 0.3 of the anode.

14. A method according to claim 1 wherein: said electric current is initially passed through said electrolyte at said normal current density and voltage values for a period of time sufficient to establish normal operation of said process; said current density is then increased to an abnormally high value within the range of from about 25 to about percent greater than said nonnal value; and said voltage is then increased to an abnormally high value within the range of from about three to about 10 times said normal value.

15. A method according to claim 14 wherein: said initial period of time is within the range of from a few minutes to about 1 hour; said current density is then increased for a period of time within the range of from 0.l to 10 minutes; and said voltage is then increased for a period of time within the range of from 0.5 to' 5 minutes.

16. A method according to claim 1 wherein said improved operation includes reducing the production of undesired byproducts, and there is recovered from said cell a cell effluent stream having a reduced content of said undesired byproducts.

17. A method according to claim 16 wherein said fluorinatable material is ethylene dichloride, and said undesired byproducts comprise monochloro and trichloro byproducts. 

2. A method in accordance with claim 1 wherein said anode effect is deliberately induced and then eliminated prior to initiating normal operation of said cell.
 3. A method in accordance with claim 1, comprising, in combination, the steps of: initially passing said current through said electrolyte at a normal current density and a normal voltage sufficient for carrying out said process; then increasing said current density to an abnormally high value which is greater than said normal current density for a period of time sufficient to deliberately induce said anode effect at said anode; then increasing said cell voltage to an abnormally high value which is greater than said normal voltage for a period of time sufficient to eliminate said anode effect; and returning said cell voltage to a normal value sufficient to maintain said normal current density.
 4. A method in accordance with claim 3 wherein: said electric current is initially passed through said electrolyte at said normal current density and voltage values for a period of time sufficient to establish normal operation of said process; said current density is then increased to an abnormal value within the range of from about 25 to about 200 percent greater than said normal value; and said voltage is then increased to an abnormal value within the range of from about three to about 10 times said normal value.
 5. A method in accordance with claim 4 wherein: said initial period of time is within the range of from a few minutes to about 1 hour; said current density is then increased for a period of time within the range of from 0.1 to 10 minutes; and said voltage is then increased for a period of time within the range of from 0.5 to 5 minutes.
 6. A method according to claim 1 wherein said anode is a porous anode, said fluorinatable material is an organic compound, and said electrolysis process comprises passing said organic compound into the pores of said anode, and recovering fluorinated product from an effluent stream from saId cell.
 7. A method according to claim 6 wherein said anode is a porous carbon anode and said organic compound is fluorinated within the pores of said anode.
 8. A method according to claim 7 wherein said organic compound is ethylene dichloride.
 9. A method according to claim 1 wherein said anode is incrementally subjected to said abnormally high current density.
 10. A method according to claim 1 comprising, in combination, the steps of: applying an electrical potential between said anode and said cathode while said anode is not immersed in said electrolyte; immersing a first portion of said anode in said electrolyte so as to subject said portion to said abnormally high current density and induce said anode effect thereon; then immersing a succeeding portion or portions of said anode in said electrolyte so as to subject each said succeeding portion to said abnormally high current density and induce said anode effect thereon; and then subjecting said anode to said abnormally high voltage so as to eliminate said anode effect.
 11. A method according to claim 10 wherein each said portion of said anode is a small portion comprising from about 0.1 to about 0.3 of the anode.
 12. A method according to claim 1 comprising, in combination, the steps of: initially immersing only a first portion of said anode in said electrolyte so as to subject said portion to said abnormally high current density and induce said anode effect thereon; then immersing a succeeding portion or portions of said anode in said electrolyte so as to subject each said succeeding portion to said abnormally high current density and induce said anode effect thereon; and then subjecting said anode to said abnormally high voltage so as to eliminate said anode effect.
 13. A method according to claim 12 wherein each said portion of said anode is a small portion comprising from about 0.1 to about 0.3 of the anode.
 14. A method according to claim 1 wherein: said electric current is initially passed through said electrolyte at said normal current density and voltage values for a period of time sufficient to establish normal operation of said process; said current density is then increased to an abnormally high value within the range of from about 25 to about 100 percent greater than said normal value; and said voltage is then increased to an abnormally high value within the range of from about three to about 10 times said normal value.
 15. A method according to claim 14 wherein: said initial period of time is within the range of from a few minutes to about 1 hour; said current density is then increased for a period of time within the range of from 0.1 to 10 minutes; and said voltage is then increased for a period of time within the range of from 0.5 to 5 minutes.
 16. A method according to claim 1 wherein said improved operation includes reducing the production of undesired byproducts, and there is recovered from said cell a cell effluent stream having a reduced content of said undesired byproducts.
 17. A method according to claim 16 wherein said fluorinatable material is ethylene dichloride, and said undesired byproducts comprise monochloro and trichloro byproducts. 